The Mammalian Cell Resource (MCR) Core Is designed to serve the Projects comprising the Program in three major areas: (1) To characterize selected mammalian cell lines, as designated by the Project Leaders, with respect to growth characteristics, chemical and radiation sensitivities (clonogenic survival), and verification of drug-resistance phenotypes conferred by genetic markers (e.g., NEC); the MCR will then act as a repository of cryopreserved, characterized mammalian cell lines, free of mycoplasma, for all Projects, ensuring quality control throughout the Program and providing confidence for inter-Project comparsions. (2) To prepare mammalian cell extracts from specific mammalian cell lines as requested by the individual Projects. (3) To provide advice and assistance for the construction of specialized cell lines (i.e., gene knockdown and knock-out) as requested by individual Projects. Thus there are three Specific Aims of the MCR: (1) To characterize, maintain, and distribute as requested mammalian cell lines which are used by the Projects constituting the Program, for the purposes of ensuring quality control and assuring the validity of inter-Project experimental comparisons; (2) To prepare small (up to 2-liter) custom extracts for mammalian cell lines as requested by the Projects, or large (up to 50-liter) extracts from commercially purchased cells provided by individual Projects; (3) To provide advice and technical assistance as appropriate for the construction and characterization of specialized mammalian cell lines such as shRNA knock-down cell lines, gene disruption (knock-out) cell lines, and cell lines with genetic modifications (e.g., minigene-complemented cell lines, APRT- and HPRT-deficient variants, etc.).